The present invention relates to a ladder-type electric filter circuit apparatus for filtering an intermediate frequency in a variety of radio equipments, in which it comprises a plurality of electric filter units each of which includes a plurality of piezoelectric resonator elements, a plurality of terminal plates and a casing for containing the resonator elements and the terminal plates.
There is a known ladder-type electric filter circuit apparatus in which a plurality of electric filter units as mentioned above are provided in a cascade configuration on a printed circuit board by means of soldering, and input and output terminal plates and an earth plate are respectively provided with protruded connecting legs which pass through associated ones of through holes bored in desired electrical conductive passage of the board. Each of the electric filter units generally comprises a plurality of piezoelectric resonator elements and a plurality of terminal plates, these resonator elements as well as the terminal being insulatingly and in cascade contained within a casing of synthetic resin.
An example of such a conventional electric filter arrangement is shown in FIG. 1. It comprises a box-shaped casing A of synthetic resin within which a plurality of piezoelectric resonator elements B and C and two terminal plates D are contained in a superimposed or stacked and electrically insulated condition. The piezoelectric resonator element B may be connected to a series branch of a ladder-type electric filter circuit and the piezoelectric resonator element C connected to a parallel branch of the ladder-type electric filter circuit. From the point of view of correctly maintaining shape and strength and executing molding, the casing A should be formed with at least about 0.3 mm in thickness. This in turn results in the increased thickness of the electric filter units and thus the increased size of a filter circuit arrangement constructed by such electric filter units.
Further, since the casing A is made from synthetic resin material which rejects a connecting means such as a soldering means, the electric filter assemblies are supported merely by the connection of the connecting legs of the respective terminal plates D in each filter assembly with the printed circuit board, thus eventually weakening the connecting strength and resulting in the unstable mounting.
Another examples of the conventional electric filter arrangement having the casing of synthetic resin are disclosed in U.S. Pat. Nos. 4,353,045 and 4,323,865.
Recently, there is an increasing demand for materializing smaller and thinner dimensions of the radio equipment incorporating such a filter circuit apparatus. These circumstances have been constituted competition in production of thiner filter circuit arrangements having a thickness of 3 mm at maximum. However, since the conventional filter arrangements heretofore proposed have usually a height of about 7 mm, they can not fully satisfy the demamd mentioned above. It has been also attempted to decrease the height of the filter arrangement by mounting it on the printed circuit board by means of a face-contacting, but this has not yet been attended with a desired result.